1. Field of Invention
The present invention relates to the technology of Radio Frequency Identification (RFID) or wireless power transmission and feedback, and more particularly to a circuit for signal decoding in RFID or wireless power charging.
2. Related Art
Wireless charging technology is a technology for charging a device without a cord and by magnetic means. The wireless charging technology originated in the wireless power transmission, which uses magnetic resonance for transmitting charges from a charger to a device through the air, wherein the coil and the capacitor is resonated between the device and the charger, to achieve high efficient energy transmission. Wireless charging technology includes the characteristics of more safety, no expose connector, no electrical leakage, and so on. Therefore, the problems of the wired charging are prevented.
As development of the Wireless charging technology, Wireless Power Consortium is established in response to the current situation. The significance of Wireless Power Consortium is to promote Qi standard. Because of the standardization, the wireless charging standard can be efficiently promoted. In Qi standard, it mentioned that in the charge process, the signal of the L-C resonance element must be decoded to serve as the signal for controlling output power and sequence procedure. Thus, the correctness of the signal transmission is very important.
FIG. 1A illustrates a circuit diagram depicting a standard transmitter of Wireless Power Consortium in the conventional art. FIG. 1B and FIG. 1C respective illustrate circuit diagrams depicting standard receiver of Wireless Power Consortium in the conventional art. Referring to FIG. 1A, the circuit of the transmitter includes a half-bridge converter 101, a resonant circuit 102, a control circuit 103 and a decoding circuit 104. In addition, referring to FIG. 1B, the circuit of the receiver includes a coil L101, a bridge rectifier B101, a receiver capacitor C101, a rectifier capacitor C102, a modulation resistor R101, a transmission switch SW101 and a communication circuit COMM. The coupling relationship of the circuit is as shown in FIG. 1B.
The receiver can be served as a mobile device, such as a mobile phone, with a wireless power charging circuit or RFID device, such as EasyCard. When the coil L101 of the receiver receives a transmitted magnetic power, after a rectifying process, the rectified magnetic power is transmitted to an integrated circuit of a mobile device or a RFID device, the mobile device or the RFID device controls the transmission switch SW101 through the communication circuit COMM. For example, if the code to be transmitted by the communication circuit COMM is “1”, the communication circuit COMM would control the transmission switch SW101 to a conduction state, and if the code to be transmitted by the communication circuit COMM is “0”, the communication circuit COMM would control the transmission switch SW101 to a cut-off state.
When the transmission switch SW101 is in the conduction state, the response of the resonant circuit 102 is the reduction of quality factor. Therefore, the amplitude of the sinusoid wave of the resonant circuit 102 would be reduced. When the transmission switch SW101 is in the cut-off state, the quality factor of the resonant circuit 102 is increased. In response thereto, the amplitude of the sinusoid wave of the resonant circuit 102 is increased. The transmission method in the wireless communication is so-called Amplitude Shift Keying (ASK).
Next, referring to FIG. 1C, the circuit of the receiver also includes a coil L101, a bridge rectifier B101, a receiver capacitor C101 and a rectifier capacitor C102. The difference between the FIG. 1B and FIG. 1C is that the modulation resistor R101 is excluded from the circuit, and a first modulation capacitor C103, a second modulation capacitor C104, a first transmission switch SW102, a second transmission switch SW103 and communication circuit COMM are included in the circuit of FIG. 1C. The coupling relationship is shown as FIG. 10.
Similarly, the receiver can be served as a mobile device, such as a mobile phone, with a wireless power charging circuit or RFID device, such as EasyCard. When the coil L101 of the receiver receives a transmitted magnetic power, after a rectifying process, the rectified magnetic power is transmitted to an integrated circuit of a mobile device or a RFID device, the mobile device or the RFID device controls the first and the second transmission switches SW102 and SW103 to perform the code transmission through the communication circuit COMM. It is to be observed that the elements coupled to the transmission switches are capacitors. When the switch(s) is/are in conduction state, the resonant frequency of the resonant circuit would be shifted. Therefore, the gain of logic “0” would be different from the gain of logic “1”. Because the code transmission is similar to that of the abovementioned method, the detail description is omitted.
Furthermore, the transmitter of the wireless charger or RFID circuit includes two control modes. The first control mode is frequency variation control mode. Generally speaking, the control signal for controlling the half bridge converter 101, which is outputted from the control circuit, is a particular pulse width modulation (PWM) signal. The PWM signal has a fixed duty cycle. When it is in light load, the frequency of the PWM signal is reduced. When it is in heavy load, the frequency of the PWM signal is increased. Since the system frequency is frequently varied, the quality factor thereof is varied, and the code in the normal communication would be hardly to be identified. The communication would be affected by loading, inductance, communication capacitance or the coil position.
When the load is heavy, the frequency offset results in the system frequency tends to the resonant frequency. In the mean time, performing the data transmission through the coil may cause decoding error if only one standard decoding circuit provided by Wireless Power Consortium is adopted to decode the transmission data.
The other control mode is to fix the duty cycle and frequency of the PWM signal of the half bridge converter 101 and to change the input voltage of the half bridge converter 101 according to the load. When the load is light, the input voltage is reduced. When the load is heavy, the input voltage is increased. Likewise, when the load is heavy, the data of the receiver received by the transmitter varies too large. The variation thereof exceeds the I/O limitation of the analog circuit in the decoding circuit. Therefore, the decoding error would occur if only one standard decoding circuit provided by Wireless Power Consortium is adopted to decode the transmission data.
Applicant implements the standard decoding circuit in the wireless charger and sent it to the Qi standard test. In five standard test coils, three circuits of standard test coils cannot pass the test. Moreover, Applicant uses the wireless charger with the standard decoding circuit provided by Qi to perform the experiment of the charging test for mobile device. During the experiment, when the load is heavy or the position between the mobile device and the wireless charger has coil offset, the mobile device would sequentially shows “charging state”, “offline state”, “charging state”, “offline state” . . . and so on. Meanwhile, the mobile device would cyclically turn on and then turn off the screen.